In the production of optical fibers, a resin coating is applied immediately after drawing of the glass fibers for protection and reinforcement of the glass fiber. Generally, two coatings are applied, a soft primary coating layer of a flexible resin (low modulus and low Tg) which is coated directly on the glass surface and a secondary coating layer of a rigid resin (higher modulus and higher Tg) which is provided over the primary coating layer. Often, for identification purposes, the fibers will be colored. Accordingly, the fibers may further be coated with an ink, which generally is a curable resin comprising a colorant (such as a pigment and/or a dye), or the secondary coating may be a colored secondary coating (i.e, comprise a colorant).
Several coated (and optionally inked) optical fibers can be bundled together to form a so-called optical fiber ribbon, e.g., four or eight coated (and optionally inked) optical fibers are arranged on a plane and secured with a binder to produce a ribbon structure having a rectangular cross section. Said binder material for binding several optical fibers to produce the optical fiber ribbon structure is called a ribbon matrix material. In addition, a material for the further binding of several optical fiber ribbons to produce multi-core optical fiber ribbons is called a bundling material.
Resins that cure on exposure to radiation such as ultraviolet radiation are favored in the industry, due to their fast cure, enabling the coated fiber to be produced at high speed. In many of these radiation curable resin compositions, use is made of urethane oligomers having reactive terminal groups (such as an acrylate or methacrylate functionality, herein referred to as (meth)acrylate functionality) and a polymer backbone. Generally, these compositions may further comprise reactive diluents, photoinitiators, and optionally suitable additives.
It is a continual objective of the industry to improve the performance of the coatings. Among the many performance characteristics required of the coating systems, the tensile strength, modulus and elongation are important. Accordingly, formulators add components to the composition to manipulate these characteristics.
The applicants have discovered that they can introduce a free multi-functional isocyanate either directly mixed with the multi-functional acrylate or into the final composition, prior to curing, and thereby improve tensile, elongation, and/or modulus properties in the composition. Applicants have furthermore discovered that the addition of relatively small amounts of aromatic urethane acrylate components can also give improved mechanical properties.